Archive for VIRGO

The New Wave of Gravitational Waves

Posted in The Universe and Stuff with tags , , , , on December 4, 2018 by telescoper

I think it’s very sneaky of the LIGO Scientific Collaboration and the Virgo Collaboration to have released two new gravitational wave papers while I was out of circulation fora  couple of days, so I’m a bit late on this, but here are links to the new results on the arXiv.

You can click on all the excerpts below to make them bigger.

First there is GWTC-1: A Gravitational-Wave Transient Catalog of Compact Binary Mergers Observed by LIGO and Virgo during the First and Second Observing Runs with this abstract:

Here is a summary of the properties of the binary systems involved in the events listed in the above paper:

There are several (four) events in this catalogue that have not previously been announced (or, for that matter, subjected to peer review) despite having been seen in the data some time ago (as far back as 2015). I’m also intrigued by the footnote on the first page which contains the following:

…all candidate events with an estimated false alarm rate (FAR) less than 1 per 30 days
and probability > 0.5 of being of astrophysical origin (see Eq. (10) for the definition) are henceforth denoted with the GW prefix.

The use of false discovery rates is discussed at length here as a corrective to relying on p-values for detections. The criteria adopted here don’t seem all that strong to me.

The second paper is Binary Black Hole Population Properties Inferred from the First and Second Observing Runs of Advanced LIGO and Advanced Virgo which has this abstract:

I’ve been teaching and/or preparing lectures all day today, so I haven’t yet had time to read these papers in detail. I will try to read them over the next few days. In the meantime I would welcome comments through the box about these new results. I wonder if there’ll be any opinions from the direction of Copenhagen?

UPDATE: Here’s a montage of all 10 binary black hole mergers `detected’ so far…

I think it’s safe to say that if GW151266 had been the first to be announced, the news would have been greeted with considerable skepticism!


And then there were five….

Posted in The Universe and Stuff with tags , , , , , , on November 17, 2017 by telescoper

…black hole mergers detected via gravitational waves, that is. Here are the key measurements for Number 5, codename GW170608. More information can be found here.

Here is the abstract of the discovery paper:

On June 8, 2017 at 02:01:16.49 UTC, a gravitational-wave signal from the merger of two stellar-mass black holes was observed by the two Advanced LIGO detectors with a network signal-to-noise ratio of 13. This system is the lightest black hole binary so far observed, with component masses 12+7-2 M⊙ and 7+2-2 M⊙ (90% credible intervals). These lie in the range of measured black hole masses in low-mass X-ray binaries, thus allowing us to compare black holes detected through gravitational waves with electromagnetic observations. The source’s luminosity distance is 340 +140-140Mpc, corresponding to redshift 0.07+0.03-0.03. We verify that the signal waveform is consistent with the predictions of general relativity.

This merger seems to have been accompanied by a lower flux of press releases than previous examples…

GW News Day

Posted in The Universe and Stuff with tags , , on October 16, 2017 by telescoper

Well, it has certainly been an eventful last day in India!

Over a hundred people gathered at IUCAA to see this evening’s press conference, which basically confirmed most of the rumours that had been circulating that a Gamma Ray Burst had been detected in both GW and EM radiation. I won’t write in detail about today’s announcement because (a) a really useful page of resources has been prepared by the group at IUCAA. Check out the fact sheet and (b) I haven’t really had time to digest all the science yet.

I will mention a couple of things, however. One is that the signal-to-noise ratio of this detection is a whopping 32.4, a value that astronomers can usually only dream of! The other is that neutron star coalescence offer the possibility to bypass the traditional `distance ladder’ approaches to get an independent measurement of the Hubble constant. The value obtained is in the range 62 to 107 km s-1 Mpc-1, which is admittedly fairly broad, but is based on only one observation of this type. It is extremely impressive to be straddling the target with the very first salvo.

The LIGO collaboration is over a thousand people. Add to that the staff of no fewer than seventy observatories (including seven in space). With all that’s going in the world, it’s great to see what humans of different nations across the globe can do when they come together and work towards a common goal. Scientific results of this kind will remembered long after the silly ramblings of our politicians and other fools have been forgotten.

I took part in a panel discussion after the results were presented, but sadly I won’t be here to see tomorrow’s papers. I hope people will save cuttings or post weblinks if there are any articles!

UPDATE: Here is a selection of the local press coverage.

Indian LIGO


As if these thrilling science results weren’t enough I finally managed to meet my old friend and former collaborator Varun Sahni (who was away last week). An invitation to dinner at his house was not to be resisted on my last night here, which explains why I didn’t write a post immediately after the press conference. Still, of got plenty of papers to read on the plane tomorrow so maybe I’ll do something when I get back.

Tomorrow morning I get up early to return to Mumbai for the flight home, and am not likely to be online again until Wednesday UK time.

Thanks to all at IUCAA (and TIFR) for making my stay so pleasant and interesting. It’s been 23 years since I was last here. I hope it’s not so long before I’m back again!

Gravitational Waves Flash!

Posted in The Universe and Stuff with tags , , , , on October 13, 2017 by telescoper

I got up early this morning to hitch a ride in a car to Mumbai so that I can give a talk this afternoon. We left Pune about 6am and got here about 8.30 so the trip was a quite a bit quicker than coming here! I’ll post about that and include some pictures when I get a moment, but first I’ll post a quick announcement.

There will be an announcement on Monday 16th October at 10am EDT (3pm BST; 7.30pm in Pune) by `the National Science Foundation (NSF) as it brings together scientists from the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo collaborations, as well as representatives for some 70 observatories’. Further details can be found here. The European Southern Observatory has also announced that it will be holding a press conference on Monday about an `unprecedented discovery’.

The fact that it involves LIGO, Virgo and representatives of other observatories strongly suggests that this announcement will address the subject of the rumours that were flying around in August. In other words, it’s likely that on Monday we will hear about the first detecting of a coalescing binary neutron star system with an optical counterpart. Exciting times!

I’ll be back in Pune by Monday and will probably be able to watch the announcement and will update if and when I can.

Gravitational Wave Flash

Posted in The Universe and Stuff with tags , , , on September 27, 2017 by telescoper

Inconveniently timed just before I was due to go to the pub, a new announcement has come out from the LIGO and Virgo gravitational wave detectors. This time it reports a coalescing binary black hole system detected by all three instruments. The new source is called GW170814, which indicates that the signal from it was received by the detectors on the day I returned from Copenhagen this summer!

Here’s the key figure:

The paper is here and there’s a Nature comment piece here.

I have to say that, on its own, the Virgo `detection’ looks rather marginal to me, but assuming that it is a detection this graphic shows how much it helps to localize the source compared to previous signals:

More on this in due course, perhaps, but now I’m off for a pint or two…


Posted in The Universe and Stuff with tags , , on August 30, 2017 by telescoper

Judging by by the WordPress blog statistics page, there’s been a lot of traffic here in the past week owing to my post about the rumours of a new gravitational wave source detected by LIGO (and possibly VIRGO). In the interest of completeness I’ll just post a quick update to mention that the latest Observation run at LIGO  finished as planned on 25th August, and this has been marked by an official announcement which I have taken the liberty of presenting here in full:

The Virgo and LIGO Scientific Collaborations have been observing since November 30, 2016 in the second Advanced Detector Observing Run ‘O2’ , searching for gravitational-wave signals, first with the two LIGO detectors, then with both LIGO and Virgo instruments operating together since August 1, 2017. Some promising gravitational-wave candidates have been identified in data from both LIGO and Virgo during our preliminary analysis, and we have shared what we currently know with astronomical observing partners. We are working hard to assure that the candidates are valid gravitational-wave events, and it will require time to establish the level of confidence needed to bring any results to the scientific community and the greater public. We will let you know as soon we have information ready to share.

The last two sentences can be translated roughly as “Back off, and give us time to analyse the data!”, which is not an unreasonable request. Judging by the timescale between detection and publication of the previous LIGO events, it will probably be a matter of months before a formal announcement is made.

I hope this clarifies the situation.




LIGO, Leaks and NGC 4993

Posted in Open Access, The Universe and Stuff with tags , , , , on August 23, 2017 by telescoper

No matter what the official policy may be, the more people there are in a collaboration the more likely it is that someone will let their excitement get to their head and start leaking news and starting rumours either directly or indirectly via social media. And so it came to pass last Friday that the following tweet appeared:

I didn’t comment on the time as I thought it might be unreliable – as it indeed it still may be – but now New Scientist has amplified the signal I feel I can’t really be blamed for mentioning it here.

The rumours going round identify the optical counterpart as being in the galaxy NGC 4993 , a red band image of which, from the Second Digitized Sky Survey (DSS2) is shown below:

NGC 4993 is the fuzzy blob slightly above and to the left of the centre of the image. It’s a fairly nondescript lenticular galaxy in a group that can be found in the constellation of Hydra. It lies in the constellation of Hydra, was actually first discovered by William Herschel and it is about 10 arcmin across on the sky. It’s quite nearby, as these things go, with a distance of about 124 million light years (i.e. 40 Mpc or so) and is about 14th magnitude.

If there is an optical counterpart to a gravitational wave event coming from this galaxy then that suggests it may be a coalescence of neutron stars. The black hole mergers that appear to be responsible to the three existing gravitational wave signals that are claimed to have been detected are not expected to release optical light. Confirmation of this interpretation can be found by where the Hubble Space Telescope was pointed yesterday:

Look familiar? HST was, in fact, observing a `BNS-Merger’ (which is short for `Binary Neutron Star’)…


If this rumour is true then it’s obviously exciting, but there are questions to be asked. Chief among these is how sure is the identification of the counterpart? A transient optical source in NGC4993 may have been observed at the same time as a gravitational wave signal was detected,  but the ability of LIGO to resolve positions on the sky is very poor. On the other hand, the European VIRGO experiment joined Advanced LIGO for the ongoing `O2′ observing run (which ends in a couple of days). Although VIRGO is less sensitive than LIGO having a third detector does improve the localization of the source – assuming, of course, that it detects a signal. Even in that case it certainly won’t be possible to pinpoint the GW source to within 10 arc minutes, which is the precision needed to place it definitely within NGC 4993.

Anyway, we wait and see what, if anything, has been found. If it is a claimed detection then I hope that LIGO and VIRGO will release sufficient data to enable the analysis to be checked and verified. That’s what most of the respondents to my poll seem to hope too!